Sir Willie said the 26-storey building had been 'very badly damaged' during the attack on the Baltic Exchange and repairs had only just been completed.
HSBC only occupies part of the building. The rest is leased to other financial institutions, including Japanese banks and the Saudi International Bank which occupies four floors.
A third of the windows were blown out by the Baltic Exchange bomb a year ago, but the building suffered no structural damage. Protective bomb-film was put on all the windows when the building was repaired.
'Fortunately we recently moved our treasury and stockbroking functions out of 99 Bishopsgate and we now occupy less than a third of the building. But if our computer system and banking records have been affected, the disruption could be considerable,' said Sir Willie.